Insights into the reaction pathway of n-butane conversion over HZSM-5 zeolite at low temperature

Abstract

The kinetics of n-butane conversion over HZSM-5 zeolites at low temperature were studied. The results show that n-butane conversion mainly occurs via monomolecular pathway under low n-butane pressure (<0.1 bar), while quickly transforms to bimolecular pathway with the increase of n-butane pressure. This can be explained by the lower apparent activation energy of bimolecular reaction compared with the monomolecular one. With the increase of n-butane pressure, the reaction orders for the formation of bimolecular products with respect to the n-butane pressure decrease from 2.0 to 1.0, which is caused by the change in the kinetic regime in which the kinetic parameters are being measured. Upon an increase in pressure, the Brønsted acid sites go from low coverage to high coverage. At low coverage of n-butane, the initial point for the measurement of apparent activation energies is the gas phase of the hydrocarbon and the highest point in energy is butyl carbonium ions. While at full coverage, the initial point is the carbenium ions and the highest point is the hydride transfer reaction, which follows a first order kinetic rate law with respect to the n-butane pressure.